OBJECTIVE: This study aims to identify recent population-based trends in maternal overweight and obesity and adverse outcomes. STUDY DESIGN: Statewide retrospective cohort study of birth certificate data for live singleton births to women in Utah between 1991 and 2001. RESULTS: Prepregnancy overweight and obesity increased from 25.1% in 1991 to 35.2% in 2001, a 40.2% increase (prevalence ratio [PR] 1.40 [1.37-1.43]), whereas maternal obesity at delivery rose 36.2% from 28.7% to 39.1% (PR 1.36 [1.33-1.39]). The attributable fraction of cesarean delivery in overweight and obese women was 0.388 (0.369-0.407). Statewide, among all women having a cesarean delivery in 2001, 1 in 7 is attributable to overweight and obesity. CONCLUSION: This is the first state-wide analysis of maternal obesity trends demonstrating a significant increase in maternal overweight and obesity. Overweight and obese women are at increased risk of cesarean delivery, preeclampsia, eclampsia, dystocia, and macrosomia, risks that increase as the body mass index rises.